The present invention relates to a method for downhole cementing, and more particularly, to a method for forming a pressure resistant seal using hydraulic cement to enhance the sealing properties of said cement, using a dump-bailer apparatus to convey the sealing cement slurry through the wellbore tubulars and displace the slurry at the proper depth.
It is well known by those practiced in the art of oil well drilling and completion technology to use hydraulic cement for various operations such as pressure isolation between reservoirs, mechanical support of casing and liners, prevention of lost fluid circulation to abnormally pressured reservoirs, and pressure isolation inside the well tubular and annular spaces. During the abandonment of reservoirs after the hydrocarbons have been removed, hydraulic cement is required to create a permanent and impermeable barrier, to prevent contamination of adjacent reservoirs, subterranean fresh-water aquifers, and surface waters.
The hydraulic cement slurry can be placed into the well by a plurality of devices; the scope of this invention refers to placement by means of dump bailer method and operations. This method typically involves a wireline or slickline tool used to place small volume of cement slurry, or similar material in a wellbore. Typically, the slurry is placed on a plug or similar device that provides a stable platform for the volume of cement plug. A dump-bailer tool includes a tubular chamber for storing the cement slurry and a ported valve for the slurry to discharge from the dump-bailer into the subterranean wellbore. The methods and devices of dump-bailer operations are well known to those practiced in the art and for instance from the U.S. Pat. No. 3,783,940.
Since the dump-bailer tool is typically used on completed wells, the diameter of the tubular chamber is reduced for the tool to be run through a production tubing. Moreover, the tool length is limited to the about 50 feet like for most wireline equipments and consequently, the volume of the cement slurry chamber does not exceed several gallons. On the other hand, the volume of the cement plug to be placed is typically 5 to 10 times greater, meaning that numerous runs are required, each involving retrieving the dump-bailer tool back to the surface and reloading it with a new cement slurry load. As a result, a plug placement may require two to three days.
To increase time efficiency, it would be suitable to reduce the required volume of a cement plug, thereby reducing the number conveyance trips of the dump-bailed cement slurry.
The purpose of placing a cement plug is to form a solid monolithic barrier that develops very low permeability to axial fluid migration and forms a permanent bonding force to the interior diametrical surface of the casing. This bonding force shall create a resistance force greater than the effects of subsequent hydraulic or mechanical forces acting on the cross-section or surface contact area of the plug and is equal to the product of the bonding surface to the bonding force per surface unit. Therefore, to increase the bonding surface, either the height of the plug (and consequently its volume) or the quality of the bonding has to be increased. Hence, improving the quality of the bonding surface would reduce the requirement of numerous conveyance trips.